The best rope is the same rope you would use for a rope halter. I use the cheap Wal-Mart poly rope, but it is stiffer
than a good quality rope. You will also need 4 - 1 inch welded rings.

I start with about 20 feet of rope for an average horse.  You may need to cut some off at the end, but you can’t add any
if it is too short. Larger horses would need more rope and the lengths listed below would need to be increased.    
Half the rope and tie a simple
knot in it. This is useful for
adjusting the noseband to
ensure a snug fit.
Thread the rope through a 1inch
welded ring. Tie a knot about 9
inches from first knot around the
ring. Keep the knot loose.
Cross each rope to the other side, about 9 inches
between knots, and tie the two  knots together. The
pictures show how the knots should look.  The ropes
of the knot should cross and look like an “X” .  This
part is very frustrating and can take several tries to
get right.
Knot details. Best to practice on a couple short
pieces of rope first and keep as a guide when
you try and tie the knots on the bridle.
After the noseband is tied then allow about 8
inches for the cheek piece and tie another
loose knot. Again cross ropes and tie the two
knots together again. This completes the
basic headstall.
You should have plenty of rope left to
thread through the diagonal ring on the
noseband. This is the cross under of the
bridle and uses pressure on the cheek,
the poll and then noseband to control
the horse.
At this point try the bridle on the horse. By keeping the
knots fairly loose you can now adjust the fit and then
tighten the knots. The ends of the rope should be equal but
if they aren’t now is the time to trim them equal lengths from
the ear knots. You can then finish them by adding rein rings
and splicing the rope in on itself several inches, at least 6
inches, and securing it with very heavy thread. Remember
this is what is connecting to your reins so make sure it is
very secure and strong.
Disclaimer. These instructions are here for information only. I do not recommend the use of this type of bridle on
any horse that is not already trained and calm. Or for any beginning rider. I am not responsible for misuse of
equipment by anyone that does not have professional help. Improperly securing end rings can result in reins
coming loose. I am not responsible for improper construction of equipment or any equipment breakage.
Instructions for tying a Dr. Cook type bridle out of rope.
The Knot "X"